The present invention concerns electromechanical filters having at least one pole or attenuation maximum in the neighbourhood of the pass band, and it relates to a filter structure of this type which is of simple mechanical construction, which provides easy industrial production and which ensures high quality of the transmission in the band.
Electromechanical filters consist essentially of a set of resonators, generally metal bars of cylindrical form, of which the dimensions are made such that, in a chosen mode of vibration, the said bars constitute circuits tuned to a preselected frequency. These bars are interconnected by means of couplers which generally consist of metal rods vibrating in a preset mode and providing mechanical coupling between adjacent resonators. One of the resonators, called the input resonator, is associated with a transducer which changes an electrical input signal into a mechanical displacement to excite the said bar into the selected mode of vibration; likewise, a second bar of the set, called the output resonator, is associated with a second transducer which effects the reverse conversion. The transducers are connected to the input and output terminals respectively of the filter. Such filters have been the subject of many works which have been abundantly described in the technical press and of patents. Such are the following U.S. Pat. No. 4,060,774 filed on June 14th, 1976 for: "Electromechanical band-pass filter for high frequencies" and U.S. Patent 4,100,506 filed on Nov. 17th, 1976 for: "Electromechanical filter" assigned to same Assignee as the present application.
It is known (see notably the article by R. A. JOHNSON, published in the report of the Symposium on Circuits and Systems Newton-April 1975, sponsored by the Institute of Electrical and Electronic Engineers), to create poles in the transmission characteristic by disposing, between non-adjacent bars, auxiliary couplers which will be termed "bridges" in the following text. The work which forms the subject of the aforesaid publication can be summed up as follows:
______________________________________ Number of bars between ends of Mechanical length Poles close to the the bridge of the bridge transmitted band ______________________________________ 2n 3 .lambda./4 + P.lambda. 1 pole on each side of the band 2n .lambda./4 + P.lambda. phase corrector (2 conjugate complex poles) 2n - 1 3 .lambda./4 + P.lambda. 1 pole of frequency lower than the band 2n - 1 .lambda./4 + P.lambda. 1 pole of frequency higher than the band ______________________________________
where
n is a positive integer other than zero PA1 p is a positive integer or zero, and PA1 .lambda. is the wavelength at the central frequency of the filter in the mode of vibration chosen for the bridge.
In regard to the construction, it is important to minimize the length of the bridges in order to eliminate any danger of irregularity being introduced into the band pass by any natural frequency of stray oscillation of the said bridge. It is therefore desirable to limit the length of the bridge to one quarter of three-quarters of the wavelength in the mode of vibration used (p=0).
The U.S. application Ser. No. 862,906 filed on Dec. 21st, 1977 for: "Electromechanical filter structure" now U.S. Pat. 4,163,960 and assigned to the same assignee discloses filters based on the foregoing study and means for improving the characteristic of these filters in the pass band.